As an integral part of our study on the functional organization of the nervous control of the circulation, we will investigate the modulatory influences of various neural structures on the carotid sinus baroreceptor reflex. Our research plan consists of two steps. First, we will obtain a comprehensive characterization of the neural modulation of baroreceptor reflex performance. For this purpose we will determine input-output relationships of the baroreceptor reflex with and without neural modulatory influences; we will isolate and force the carotid sinus with the servo-controlled pressure (input) while measuring various hemodynamic responses (outputs; i.e., arterial pressure, heart rate, cardiac contractility, total peripheral resistance, regional blood flows and resistances, and systemic vascular compliance). Neural modulation will be defined as the alterations in input-output relationships produced by manipulations of the neural structures. The second step will be to determine the neural site of interaction between the structures in question and the baroreceptor reflex, and further to determine the neural connections between the structures and the neural sites of interaction. For the second step, the techniques of focal electrical stimulation, discrete neural lesions, recording of action potential, and detailed histological analysis will be added to the hemodynamic characterization of the baroreceptor reflex.